There are many applications where measurements are being taken on a particular signal during a predetermined time interval, the time interval being controlled by a measurement gate signal. For example, in a system for measuring the frequency and other characteristics of a pulsed or continuous wave microwave signal, a measurement gate may be utilized to assure that measurements are taken within the pulsed microwave signal and that the measurement gate is sufficiently spaced from the beginning and end of the microwave pulse so that turn-on and turn-off transients will not corrupt the measurement. Where there is a change in frequency during a microwave signal or pulse, such as for a radar chirp, it is also important to know where in the signal readings are being taken so that a frequency profile of the signal can be generated.
However, because of variations in internal delays in the system, such as in counters or pulse generators, or in the cabling used with the system, it is possible that the measurement gate may fall outside of the microwave pulse being measured resulting in inaccurate readings. Such potential errors are of concern to the user, but are currently difficult to identify. Similarly, current systems do not afford the user an easy way of verifying where in a chirped or frequency varying signal a particular set of measurements are being taken, or in other words where the measurement gate is positioned relative to the signal being measured.
The above problems exist both where (a) the measurement gate is of fixed duration and measurement is effected by counting the number of cycles or events during the fixed time interval; and (b) a fixed number of event or cycles for the signal being measured determines the gate width and the duration of the gate is determined to measure frequency. The later method will sometimes be referred to as the reciprocal counting technique. The problems also exist both where the measurement gate is being automatically generated, for example in response to predetermined signal conditions, and where the gate is being generated in response to an external user-controlled input. The user-controlled input provides greater flexibility in determining both the location and duration of the measurement gate. However, absent some visual feedback as to where the measurement gate is occurring relative to the signal being measured, and in view of the variable delays and synchronizations which can occur in such measurement systems which make it difficult to maintain controls accurately calibrated, it is extremely difficult for a user to achieve precision manual positioning of a measurement gate.
In view of the above, a need exists for apparatus affording a user an accurate visual indication of the position of a measurement gate relative to the signal being measured so that a user may verify that the measurement gate is properly positioned relative to the signal to be measured and make adjustments if necessary.